What You Think You Know About Sept. 11 …

… but don't.

The Saudi government paid off al-Qaida in exchange for immunity from terror attacks. Saudi princes knew in advance about the Sept. 11 attacks. Most of the Saudi officials who assisted al-Qaida all died mysteriously soon thereafter. The revelations in Gerald Posner's new book Why America Slept are an astonishing reminder of just how much we still don't know about Sept. 11 and its planning.

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But there is also plenty that we think we know but don't. I'm not talking about shoddy conspiracy theories (that Jews were warned not to show up for work at the World Trade Center, for example) believed by the ignorant and the paranoid, but widespread misconceptions held by everyday Americans. Here are six of the most common:

David Plotz is the CEO of Atlas Obscura and host of the Slate Political Gabfest.

1. The misconception: Zacarias Moussaoui was the "20th hijacker." In the first months after the attacks, federal officials—including Vice President Cheney—hinted that Moussaoui, who was taken into federal custody before Sept. 11, might have been the missing man on the Flight 93 hijacking team. Moussaoui's indictment in Dec. 2001 also linked him to the Sept. 11 plot, trying to show parallels between Moussaoui and the Sept. 11 terrorists—flight training, joining a gym, mysterious funding from overseas, connection to ringleader Ramzi Binalshibh, etc.

What's wrong with the story: There is no actual evidence that Moussaoui was supposed to be on Flight 93 or the other planes. Moussaoui had no contact with any of the Sept. 11 hijackers and took his flight training long after they did. According to Yosri Fouda and Nick Fielding's Masterminds of Terror, Binalshibh has said that while he contemplated Moussaoui as an understudy for 9/11, he was never part of the plot. Binalshibh said he was glad that he kept Moussaoui, who was not really trusted by al-Qaida, away from the other hijackers. (Incidentally, it is Binalshibh who was a failed hijacker: He couldn't get a U.S. visa.) This does not excuse Moussaoui, a truly bad guy who was apparently preparing for some act of airplane terrorism.

(Bonus Moussaoui misconception: that he only wanted to learn how to steer jumbo jets, not take off or land. In fact, as this SlateExplainer notes, the opposite is true: Moussaoui only wanted to learn takeoffs and landings.)

2. The misconception: We know how the hijackers seized the planes. Within days of Sept. 11, Americans believed they knew how the planes were grabbed: Terrorists had taken control by stabbing pilots, passengers, and flight attendants with box cutters and knives.

What's wrong with the story: It's incomplete and misleading. We don't really know what happened on the planes. The cockpit voice recorder survived neither New York crash and was damaged beyond salvage in the Pentagon crash. The Flight 93 voice recorder doesn't start until several minutes after the hijackers took the plane. What little we know about tactics and weapons comes from phones calls made by passengers and flight attendants. As Edward Jay Epstein has pointed out, the evidence is incredibly paltry. No one on United Flight 175, which crashed into the World Trade Center, reported anything about weapons or tactics. One flight attendant on American Flight 11, which also crashed into the World Trade Center, said she was disabled by a chemical spray, while another flight attendant said a passenger was stabbed or shot. On the Pentagon plane, American Flight 77, Barbara Olson reported hijackers carrying knives and box cutters but did not describe how they took the cockpit. And on United Flight 93, passengers reported knives but also a hijacker threatening to explode a bomb. The box cutter-knives story isn't demonstrably false, but it serves to divert attention from the other weapons and to mask the fact that we don't have any idea how the hijackings happened.

3. The misconception: Iraq was involved in the Sept. 11 attacks. According to an August Washington Post poll, nearly 70 percent of Americans believe Iraq played a role.

What's wrong with the story: For starters, the two captured planners of the 9/11 attacks, Binalshibh and Khalid Sheikh Mohammed, have both reportedly denied Iraqi involvement during interrogations. Next, those who argue for Iraq's guilt rely on dubious claims. The first is an on-again, off-again Czech assertion that Mohamed Atta met with an Iraqi agent in Prague. But American intelligence agencies now believe the meeting did not occur. (This Slate dialogue debated the Atta meeting and other Iraqi links to terrorism.) Several conservative analysts—notably Laurie Mylroie and former CIA Director James Woolsey—have pushed the idea that the first World Trade Center bombing was an Iraqi intelligence operation, and thus Sept. 11 might have been too. They believe that Ramzi Yousef, the architect of the first bombing, was acting for the Iraqis, and since Yousef's uncle is Khalid Sheikh Mohammed, Iraq should be suspect again. But no one has managed to show that Iraq sponsored Ramzi Yousef or the 9/11 terrorists.

Perhaps the most compelling evidence against Iraqi involvement is that the Bush administration hasn't made a case for it. The president is desperate to link Iraq to al-Qaida. But so far, his team hasn't managed to find anything tangible that connects the Hussein regime to Osama Bin Laden (much less to 9/11). The administration wants the nefarious alliance so much that if it had any evidence, it surely would have leaked it. This does not prove, however, that Iraq and al-Qaida never cooperated. The polls, in fact, may reflect a kind of commonsense logic: Saddam Hussein and al-Qaida share a pathological hatred of the United States, so it's entirely possible that they collaborated, even if we don't know how.

4. The misconception: The Sept. 11 plotters planned to use crop-dusters for a biological or chemical attack.

What's wrong with the story: On the surface, the case for crop-dusters is powerful. The federal government twice grounded crop-dusters after 9/11 because of suspicion they might be used for attacks. The original indictment of Moussaoui suggested that he and the 9/11 plotters were investigating crop-dusters. Workers at a crop-dusting company in Florida reported that Mohamed Atta and other Arab men repeatedly inquired about crop-dusters. A Department of Agriculture official named Johnelle Bryant claimed that Atta visited her in early 2000 and asked for a government loan to buy a plane that he would modify for crop-dusting.

But as Edward Jay Epstein has pointed out, the crop-dusting stories are squirrelly. A crop-dusting worker claimed Atta dropped by the weekend before 9/11, but Atta had already left Florida. Bryant pinpointed Atta's visit to late April or early-mid-May of 2000—but this was before Atta even arrived in the United States. When prosecutors revised the Moussaoui indictment in 2002, they also dropped all mention of crop-dusting. And in interviews with Al Jazeera's Yosri Fouda, Ramzi Binalshibh and Khalid Sheikh Mohammed didn't mention any interest in a crop-dusting attack. They indicated that the plan was always to fly airplanes into buildings.

Some al-Qaida operatives may have inquired about crop-dusting, and one may even have sought a loan from Johnelle Bryant. (Some terrorism analysts speculate that before al-Qaida decided to seize airliners, it planned to buy a small plane, fill it with explosives, and crash it.) The crop-dusting story can't be disproved, but no solid public evidence exists that the 9/11 plotters were interested in either crop-dusters or a biological or chemical attack.

5. The misconception: Terrorists or their supporters profited by speculating on airline stocks before 9/11.

What's wrong with the story: Terrorists may have profiteered, but the evidence is sketchy. As was widely reported after 9/11, the options market for United and American Airlines was unusually busy in the days before 9/11, with an extremely heavy volume of "put options"—bets that the airline shares would fall. By the end of September 2001, both the Chicago Board Options Exchange and the Securities and Exchange Commission had launched investigations into the unusual trading. Since then, they've been silent. Two years later, neither the exchange nor the SEC will comment on its investigation. Neither has announced any conclusion. The SEC has not filed any complaint alleging illegal activity, nor has the Justice Department announced any investigation or prosecution.

This does not mean terrorist wagering didn't occur: It might well have. The absence of any complaint suggests the SEC found nothing illegal, but that's not definite. The SEC and the Chicago board seal the records of their investigations and won't offer any explanation—even if there is an innocent one—for the strange trading. So, unless the SEC decides to file a complaint—unlikely at this late stage—we may never know what they learned about terror trading.

6. The misconception: No one could have predicted the Sept. 11 attacks. Since 9/11, President Bush and his team have repeatedly insisted that the attacks were inconceivable. David Corn chronicles these claims in his new book The Lies of George W. Bush: Mastering the Politics of Deception. In May 2002, for example, Condoleezza Rice said, "I don't think anyone could have predicted that these people would take an airplane and slam it into the World Trade Center." Ari Fleischer echoed her, "Never did we imagine what would take place on Sept. 11 where people use those airplanes as missiles and weapons."

What's wrong with the story: In fact, there were tons of warnings of exactly this kind of attack. The recent congressional report on the 9/11 intelligence failures lists a dozen pre-9/11 indications that terrorists were plotting a suicide hijacking. For example, in 1994 Algerians hijacked an Air France airliner with the intention of crashing it into the Eiffel Tower. (They were tricked by French officials into landing in Marseilles to refuel, where they were overpowered.) In 1995, police in the Philippines uncovered an al-Qaida plot to fly a plane into CIA headquarters. (One of the plotters: Khalid Sheikh Mohammed.) A year later, al-Qaida had the idea of flying a plane from outside the United States and crashing it into the White House. Two years later, al-Qaida planned to fly a plane from outside the United States and crash it into the World Trade Center. And so on.

Intelligence officials, who are endlessly juggling all kinds of different threats, didn't take the suicide-plane schemes seriously because they believed there were other, more imminent dangers. But no one can say they weren't warned.